Marine seismographic prospecting



March 17, 1970 s. PATERSON 3,500,949

MARINE SEISMQGRAPHIC PROSPECTING Filed Dec. 18, 19s? 3 Sheets-Sheet 1554% gig,

March 17, 1970 s. PATERSON 3,

MARINE SEISMOGRAPHIC PROSPECTING Filed Dec. 18, 1967 3 Sheets-Sheet 2arch 17, 1970 Filed Dec. 18, 1967 S. PATERSON MARINE SEISMOGRAPHICPROSPECTING FIG. 6

3 Sheets-Sheet 5 US. Cl. 181.5 6 Claims ABSTRACT OF THE DISCLOSUREUnderwater seismic prospecting is carried out by detonating an immersedbubble-forming explosive having a length-to-diameter ratio exceeding :1thereby suppressing bubble pulsation which would normally occur unlessthe explosive is located close to the water surface.

This invention relates to a method of marine seismographic prospecting,and to explosive charges for use therein.

In marine seismographic prospecting pressure waves are generated byfiring explosive charges immersed in water, and the arrival at a ship ofthe pressure waves reflected from underwater rock strata are recorded onseismographs on the ship. When an explosive charge is fired at a greatdepth, about half the explosive energy is usefully transformed into aspherically expanding shock wave. After this has been radiated, theremaining half of the energy still remains largely in the product gases,whose pressure greatly exceeds the ambient hydrostatic pressure. Inconsequence, the bubble of gas expands, setting the water into outwardmotion. Because of the inertia of the water, this process does notterminate when the bubble pressure has fallen to the ambient pressure,but instead an overshoot takes place. In due course, the water comes torest, with the bubble now greatly enlarged and at a very low pressure.Gravitational force now reverses the motion, and the bubble is rapidlycontracted again, overshooting once more takes place and the bubble iscompressed to a high pressure and a relatively small volume. As themotion is again reversed, a second pressure pulse is radiated outwards.This entire cycle is repeated several times with decreasing amplitude,and a series of pulses is radiated. The peak pressure in the firstpulse, although small compared with that in the primary shock, is ofmuch longer duration and, in consequence, the momentum and energytransferred are not negligible when compared with those of the primaryshock. This first bubble pulse is received by the recording equipmentduring the time interval in which the reflected signals are arriving andthe record can become confused. In order to suppress bubble pulsation,one can work with charges quite close to the surface, and this is whatis done at present. Release through the surface then suppresses bubbleoscillations, but it also greatly reduces the intensity of the outgoingshock wave. In typical cases, instead of radiating one half theexplosive energy in the shock, only oneeighth may be thus radiated.Elimination of the bubble action would permit firing at greater depthsand could, therefore, lead to as much as a four-fold increase inefliciency.

In accordance with the present invention, a method of seismographicprospecting for producing a record of underwater rock strata comprisesgenerating a pressure wave by the firing underwater of an explosivecharge having a length to diameter ratio exceeding 5:1 and recording theresulting seismic waves after reflection or refraction from underwaterrock layer interfaces. Preferably the length to diameter ratio isbetween l and 100:1 since such charges may contain suflicient charge perunit length to ates ac give a strong pressure wave whilst still beingconvenient to handle. Heavy charges having a length to diameter ratioexceeding :1 are cumbersome to work with in prospecting conditions. Thecharge distribution preferred is from 0.2 to 5 lb. per foot.

The invention also includes waterproof charges of detonatable explosivefor use in underwater seismographic prospecting having a length todiameter ratio exceeding 5:1. It will be obvious that the explosivecharges need not be cylindrical and that diameter in this context isused in a wider sense as being the maximum cross-sectional dimension.The explosive charge may be a unitary charge comprising explosive in arigid container such as, for example, a tinplate canister or a tubularcardboard container, or the explosive charge may be contained in aflexible tube of, for example, synthetic thermoplastic which may besupplied in coiled form in bulk. Alternatively, the explosive charge maybe an assembly of spaced charges connected for substantiallyinstantaneous initiation and assembled so that the overall length todiameter ratio of the assembled charge exceeds 5:1. It will be apparentthat the smaller charges in the assembly may or may not have a length todiameter ratio exceeding 5:1. The charges may, for example, be connectedtogether for initiation by a line of detonating fuse-cord.

The invention further includes an apparatus assembly for seismicprospecting, com-prising an explosive charge of the invention locatedunderwater and means for recording a seismic wave reflected or refractedfrom an underwater rock layer interface, consequently on detonation ofthe explosive charge.

When marine seismographic prospecting is carried out in accordance withthe method of the invention, using an elongated charge, the bubble pulseis much diminished in comparison to the bubble pulse obtained using thecharges of the same weight and of the squat shape normally used, whereasthe primary shock wave is not substantially different. The bubble pulsesuppression effect is more marked when the charges are orientatedhorizontally on firing.

The invention is further illustrated by the following description ofexamples of explosive charges designed for use in putting the inventioninto practice. In the description reference is made to the accompanyingdrawings in which:

FIG. 1 shows a diagrammatical longitudinal section of a canister primercartridge,

FIG. 2 shows a diagrammatical longitudinal section of a canistermain-charge cartridge,

FIG. 3 shows a diagrammatical longitudinal section of an elongatedflexible cartridge assembly,

FIG. 4 shows a bundled assembly of the cartridges shown in FIG. 3,

FIG. 5 is a sectional view on the line VV of FIG. 4,

FIG. 6 shows a spaced assembly of the cartridges shown in FIG. 3.

EXAMPLE 1 The canister primer cartridge shown in FIG. 1 has an elongatedtubular tinplate body 11, sealed at one end with a recessed tinplate cap12 which is formed with a pocket 13 to accommodate a detonator and atthe other end with a tinplate cap 14 formed with an internalscrew-threaded portion 15 adapted for attachment by screw engagement toa screw-ended canister cartridge of a less sensitive main explosivecharge. The cartridge is filled with a charge of powdered explosiveconsisting (by weight) of 88.5% of ammonium nitrate, 3.5% of anthraciteand 8% of trinitro toluene.

The canister cartridge of main explosive charge shown in FIG. 2 has atubular tinplate body 16 sealed with tinplate end caps 17, 18 formedrespectively with matching male and female threads. The cartridge isfilled with a charge of powdered explosive consisting (by weight) of 92%of ammonium nitrate and 8% of trinitrotoluene.

The bodies of both cartridges are inches long and 2 inches in diameter.In use, the primer cartridge and optionally one or more cartridges ofmain charge attached thereto, are immersed in the water over the area tobe surveyed and fired by an electric detonator disposed in the detonatorpocket 13. The length to diameter ratio of the assembled charge may bevaried at will by variation in the number of cartridges.

EXAMPLE 2 The explosive cartridge shown in FIG. 3 has an elongatedtubular polyethylene body 19, 25 inches long and having an internaldiameter of 0.625 in., and a wall thickness of 0.1 in. A polyethylenesleeve 20 is closely fitted over one end of the cartridge and thissleeve is used to attach a detonator or primer or to attach a furthersimilar tubular charge to vary the length of the charge. The cartridgeis filled with an explosive charge which may, for example, have thecomposition of the primer or main charge given in Example 1.

EXAMPLE 3 In the assembly shown in FIG. 4, four of the cartridges 21 asshown in FIG. 3 are bundled together and held by strap 22. The explosiveloading is thus readily varied by varying the number of cartridges inthe bundle.

EXAMPLE 4 In the charge assembly shown in FIG. 6 a number of cartridges23, as shown in FIG. 2, are spaced apart along a rope 24- and attachedthereto by tapes 25. Detonators 26 are attached one to each cartridge bysleeves 27.

The cartridges shown in FIG. 1 may also be used in a spaced assemblywherein each cartridge is provided with a detonator.

What I claim is:

1. In the method of seismographic prospecting for producing a record ofunderwater rock strata, said method being of the type which includesfiring an explosive immersed in the water thereby forming a bubble ofcombustion gases and recording the resulting seismic waves afterreflection or refraction from the underwater rock layer interfaces, theimprovement comprising predetermining parameters for a suitable firingdepth for an elongated condensed explosive charge having a length tomaximum cross sectional dimension ratio exceeding 5:1, saidpredetermining step including selecting a depth at which a generalizedshock wave will be utilized to provide energy from the shock pulse fordirecting the wave toward the rock strata and to control the bubbletravel in a manner to prevent bubble breakthrough at the water surface,thereby providing substantially greater energy from the shock pulse thanis provided under conditions where bubble breakthrough does occur;submerging the charge to at least said predetermined depth; and firingthe charge, whereby undesired multiple pressure pulses of the typeusually associated with explosion bubbles which do not break through thewater surface are suppressed.

2. A method as claimed in claim 1 wherein the length to diameter ratioof the explosive charge is between 10:1 and :1.

3. A method as claimed in claim 1 wherein the explosive chargedistribution is from 0.2 to 5 lb./sq. in.

4. A method as claimed in claim 1 wherein the explosive charge iscontained in a container the walls of which are constructed of amaterial selected from the group consisting of tinplate, cardboard andsynthetic thermoplastic.

5. A method as claimed in claim 1 wherein the explosive charge comprisesan assembly of spaced charges connected for substantially instantaneousinitiation.

6. A method as claimed in claim 5 wherein the spaced charges areconnected together for initiation by detonating fuse-cord.

References Cited UNITED STATES PATENTS 2,675,882 4/1954 Bazzoni et all81.5 2,842,056 7/1958 Klotz 18l.5 3,009,526 11/1961 Andrews et a1.l81.5 3,256,501 6/1966 Smith 34012 X 3,326,126 6/1967 Berthmann et al.l81.5 2,619,186 11/1952 Carlisle 181.5

BENJAMIN A. BORCHELT, Primary Examiner T. H. WEBB, Assistant ExaminerU.S. Cl. X.R. 340-5, 12

